Among various semiconductor elements, the amount of heat generated is large in a high power control power element, which is used for controlling electric automobiles, electric vehicles and the like, for example. As a substrate on which one of these high power control power elements is mounted, a power module substrate is widely used in the past. In the power module substrate, a highly conductive metal plate is bonded on a ceramics substrate such as one made of AlN (aluminum nitride) as a circuit layer.
In the conventionally used power module substrate, the semiconductor element that is the power element is mounted on the circuit layer through a solder material (for example, refer Patent Literature 1 (PTL 1)).
As the metal constituting the circuit layer, aluminum, aluminum alloy, cupper, and copper alloy are commonly used.
When the circuit layer is made of aluminum, it is hard to bond to the solder material effectively since the naturally formed oxide layer is formed on the surface of aluminum. In addition, when the circuit layer is made of copper, there is a problem that the conductivity of the circuit layer is deteriorated due to penetration of the composition of the solder material in the circuit layer after reaction between the melted solder material and copper.
On the other hand, as a bonding method free of the use of the solder material, the method, in which the semiconductor element is bonded by using a Ag nanopaste, is proposed in Patent Literature 2 (PTL 2), for example.
In addition, methods, in which the semiconductor is bonded by using oxide pastes including metallic oxide particles and a reducing agent made of an organic compound without using the solder material, are proposed in Patent Literatures 3 and 4 (PTL 3 and PTL 4), for example.
However, when the semiconductor is bonded by using the Ag nanopaste without using the solder material, a bonding layer of the Ag nanopaste thinner than one formed with the solder material is formed as disclosed in PTL 2. Therefore, the stress during loading thermal cycles becomes more likely to be placed on the semiconductor and the semiconductor itself is broken down occasionally.
Similarly, when the semiconductor is bonded by using the metallic oxide and the reducing agent, the sintered layer of the oxide paste is thinly formed as disclosed in PTLs 3 and 4. Therefore, the stress during loading thermal cycles becomes more likely to be placed on the semiconductor and the performance of the power module is deteriorated occasionally.
Under the circumstance explained above, methods, in which the circuit layer and the semiconductor are bonded through the solder material or the Ag paste after forming a sintered Ag layer on the circuit layer made of aluminum or copper by using a glass-containing Ag paste, are proposed in Patent Literatures 5-7 (PTLs 5-7), for example. In these methods, the glass-containing Ag paste is applied on the surface of the circuit layer made of aluminum or copper; and subjected to sintering. By following the process, the oxide film formed on the circuit layer is removed since the oxide film reacts with the glass; and the sintered Ag layer is formed. Accordingly, the semiconductor element is bonded on the circuit layer, on which the sintered Ag layer is formed, through the solder material.
This sintered Ag layer has the glass layer, which is formed by the reaction between the glass and the oxide film of the circuit layer; and the Ag layer formed on the glass layer. In this glass layer, conductive particles are dispersed; and conduction of the glass layer is retained by these conductive particles.